Countdown Timer buzzer

[Stevie_j_b]

Hi all,

I'm posting here in the hope that someone can help me, and hopefully I can elaborate the subject out for you sufficiently!

I work for a design and development company, and we deal with a wide variety of work, including recreations of training aids. Something has come into the office which we are to reverse-engineer, in a bid to reproduce (and simplify) for repeated production. Basically, this device functions as a training aid. The device contains a digital countdown timer (AA battery powered), which counts down from a set value. Upon reaching zero, the timer buzzer goes off. The intermittent buzzing isn't sufficiently loud, and needs to be constant not intermittent, and therefore the leads connecting to the speaker are spliced off into a little circuit, which activates a constant wail from a larger speaker, connected to a 9v battery.

I did a bit of searching on Google for component descriptions, and we have a thyristor, transistor, diode, and various resisters. Since I'd never covered thyristors in my Mech Eng degree course, I dug around a little, and it seems this is used to latch upon an input from the little speaker, forcing the loudspeaker to wail permanently. I had a feeling the transistor is used to drive up the output from the little speaker to above the threshold (1.5v) on the thyristor gate, so I had a play, but while the larger speaker does beep, it does so intermittently, in sync with the smaller speaker.

I'm sure this is quite a simple circuit I need, but my basic level of knowledge has forced me into a dead end! Is there anyone who understands what I need, and has any direction for me? I can try and give you more information if it's required, so let me know!

Thanks in anticipation,

Steve

 

[Russlk]

A 9 volt battery is not designed to produce much power. I expect that you are using 3 AA cells to power the logic, so add 3 more to get 9 volts total. The existing buzzer is not loud enuf, so take it out and use that signal to fire the SCR/TRIAC. A 100dB buzzer connected to the 9 volts should be loud enuf. How loud do you want?

 

[Stevie_j_b]

I've directly connected the louder buzzer up to the 9v battery, and it is sufficiently loud.

The problem I have is to design the circuitry to take the timer buzzer sound (Like a digital watch alarm in decibel level), and use this signal to latch a thyristor to continuously sound the buzzer connected to the 9v supply.

We know the device works, using whatever wiring the original did, but trying to copy the wiring setup has proved a futile exercise! So, we are trying to reproduce the circuit, knowing that there is a thyristor in there somewhere! I've managed to get the larger buzzer to activate in time with the smaller buzzer, but this produces an intermittant signal, not latching. The best thing to explain this as is a detonator circuit, where a small current (small buzzer) latches a larger circuit (large buzzer), connected via seperate power supplies.

Thanks for the reply, hopefully this defines the problem more?

 

[ericgibbs]

Hi Stevie,
Do you have a rough sketch you could post?

Makes helping much easier.

EricG

 

[Stevie_j_b]

Sure,

Little embarassing, as I know this circuit is very bad, but you'll have to excuse my complete lack of reasonable knowledge!

I am using the output of the little speaker to bias the transistor, which amplifies the signal to rail, which then is used to latch the thyristor, which then applies the 9v across the large buzzer. This is what I am trying to do, but at the minute the large buzzer is just replicating the smaller buzzer signal, instead of being latched on as required.

I figured this was the method the previous system used, as the output on the small speaker must be negligible, and that alone isn't enough to latch the thyristor (rated at 1.5v). I assume there needs to be some form of resistor network in there to limit the voltage into the thyristor to something below 9v, but I'm beyond my realms of capability!

Hopefully this helps!

Thanks a lot, I appreciate it

Steve

 

[ericgibbs]

Stevie,

This circuit should be close to what you want, not knowing the voltage levels
fromthe small speaker.

Eric

 

[Hero999]

That probably won't work

It's highly likely that the buzzer will turn the SCR off because it draws a pulsed current waveform. You sould put a resistor in parallel with the buzzer to keep the current flowing in between pulses.

 

[ericgibbs]

hi stevie,

Not knowing any details on your large buzzer, Hero could be right.

Depending upon the type of buzzer, the 1K0 may be needed.

Lets Know

Eric

EDIT: In light of the OP's last post.
Hi Hero,
Sounds as though its some type of 'Electronic' buzzer, that dosnt interrupt its own supply?
Anyway, appreciate your observation ref hold ON resistor.

 

[Stevie_j_b]

Thanks for the help, it is much appreciated!

After fighting with the thyristor (the pins I thought were correct actually weren't!), the circuit worked perfectly as Eric said originally! I didn't get your version Hero999 until after I came back from the workshop, so since it works, I'll leave it as it is!

For your own benefit, the circuit is used within an imitation bomb disposal training device, along with anti-tilt and anti-release mechanism (think the end of Speed 1), and this circuit basically sounds a really loud buzzer when the disposal team are out of time!

Anyway, thanks again, it was extremely beneficial and your quick response is much appreciated!

Steve